Our lab has just published a new paper in PLoS ONE, detailing the interactions of coral and algae on the Great Barrier Reef, and uncovered just how resilient some reefs can be following coral bleaching events. The southern end of the Great Barrier Reef was exposed to extended periods of high sea surface temperatures in the end of 2006, resulting in extensive coral bleaching across the Keppel Islands throughout January 2006. Following the bleaching event, a single species of fleshy macro-algae (Lobophora) overgrew the coral skeletons, causing high rates of mortality throughout the second half of 2006. But, by February 2007, corals were rapidly recovering due to an unusual seasonal dieback of the macro-algae, and astonishing regenerative capabilities of the dominant branching Acroporid corals – almost twice the rate of offshore corals on the northern Great Barrier Reef.

What is unusual about the Keppel Islands story is threefold: first, that corals recovered within months to years (reversal of macro-algae dominated reefs often takes decades), second, recovery of the corals occurred in the absence of herbivory (traditionally assumed to be the ‘driving factor’ in macro-algal phase shifts), and third, that corals recovered through asexual (regenerative) capacities rather than reseeding of reefs by larval recruitment. Understanding the processes that drive recovery following disturbances is critical for management of coral reefs, and the Keppel Islands example shows that managing local stressors (overfishing and water quality) helps reefs bounce back from global stressors such as coral bleaching events. PLoS One is an open-access journal, so the article is free to read – click on the link below, and feel free to rate and comments on the paper. Congratulations Guillermo et al!

This is great and a pretty amazing story. As far as I know this is the first “complete” reversal of a phase shift – all other oft-cited cases do not include a full recovery to pre-shift coral cover! It reminds me of a paper (that should be getting more attention) from 2006 by Stimson and Conklin (Potential reversal of a phase shift: the rapid decrease in the cover of the invasive green macroalga Dictyosphaeria cavernosa Forsskål on coral reefs in Kāne‘ohe Bay, Oahu, Hawai‘i. Coral Reefs 27 (4): 717-726) – where decades of constant high abundances of macroalgae on the reef slopes og Kaneohe Bay declined dramatically throughout the bay during a few months of 2006. The decrease was the result of an unusually protracted cloudy, rainy period in March 2006, which reduced irradiance and caused the alga to lose weight.

Again, two cases of rapid macroalgal decline on reefs that have undergone phase shifts without any (recorded) influence of the classic driver of herbivory.

I think our Dairy Bull Jamaica story (Idjadi et al 2006) is indeed an example of a phase shift reversal. After being largely replaced by macroalgae in the 1980s and 1990, coral cover rebounded to about 55% by 2004. We don’t know what coral cover was at that site before the phase shift. It could have been higher, but I wouldn’t be surprised if it was indeed 50-70% ish. Since Caribbean reefs lack plating coral species, coral cover is rarely higher than that. In the Dairy Bull case, herbivory, namely the return of Diadema (urchins), was an important factor driving the reversal, at least in reducing macroalgal biomass. Although I doubt that urchins played much of a role the coral recovery.

Another study mentions a local case of phase-shift reversal observed in the Indian Ocean after 5 years of post-cyclone algal dominance. The reasons of this phenomenon still remain unclear for several favorable factors might have been combined.
Scopélitis et al., in press. Changes of coral communities over 35 years: Integrating in situ and remote sensing data on
Saint-Leu Reef (la Réunion, Indian Ocean). Estuarine, Coastal and Shelf Science (2009), doi: 10.1016/j.ecss.2009.04.030
(proofed version of the paper will be available soon)

Hopefully the aforementioned reversals will remain stable in time.

A recent work shows that it was unfortunately not the case at Dairy Bull where recovered corals did not survive the 2005 bleaching.
Quinn and Kojis, 2008. The recent collapse of a rapid phase-shift reversal on a Jamaican north coast coral reef after the 2005 bleaching event. Rev.Biol.Trop.(int.J.Trop.Biol. ISSN-0034-7744) Vol 56 (Suppl.1) 149-159, May 2008.